tracker-preferences: Workaround for gtk+ bug setting scalers to max default
The scaler type in gtk+ seems to be setting the scaler's value to its max default when the scaler is in a tab, the tab isn't open and gtk_widget_show_all is called for. It also signals when this happens the changed signal. In the signal handler we set the config object. If the tab is never activated that means that we've written the config value with some- thing the user didn't ask for (the default max value). I workaround-fixed this by only setting the config object on apply-click.